von Strandmann,P.A.E.P., Opfergelt,S., Lai,Y.-J., Sigfussoniu‚ฬ“ช‚ษLj,B., Gislason,S.R. and Burton,K.W.(2012): Lithium, magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in Iceland. Earth and Planetary Science Letters, 339-340, 11-23.

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wAbstract
@This study presents lithium, magnesium and silicon isotope ratios from pore waters and soils from a well-characterised Histic Andosol in south-west Iceland. The soil ƒย7Li composition ranges between values slightly lighter than basalt, to those that are much heavier (-1.1๑ to +26.8๑), and are possibly influenced by sea salt. In contrast, precipitation-corrected dissolved (pore water) ƒย7Li values (1.8-10.0๑) appear to reflect preferential adsorption of 6Li onto secondary minerals, where allophane supersaturation results in high ƒย7Li values. Conversely low ƒย7Li together with high [Li] are probably due to destabilisation of allophane at low pH, and thus desorption of Li. When compared to Icelandic river values, it would appear that soil pore waters reflect at intermediate isotope composition between basalts and river waters. Precipitation corrected pore water Mg isotope ratios (ƒย26Mg) range between -0.46๑ and -0.12๑, and correlate with the amount of heavy Mg adsorbed onto the soil exchange complex. Silicon isotopes in the soils are isotopically lighter (ƒย30Si = -0.91๑ to -0.53๑) than basalt (-0.29๑), whereas pore waters are heavier (+0.13๑ to +1.03๑). Soil ƒย30Si values show a clear evolution between unweathered basalt and a hypothetical isotopically light endmember representing secondary minerals. Dissolved Si isotopes also respond to chemical weathering processes, and show that isotopically heavy ƒย30Si corresponds to high cation fluxes and high secondary mineral formation. However, comparison of all these proposed isotopic weathering tracers suggests that they respond differently to the same chemical weathering conditions. This indicates a differing behaviour during secondary mineral neoformation or adsorption depending on whether the incorporated element is a major or trace constituent. In turn, this behaviour can potentially yield important information on secondary mineral behaviour and destabilisation, and thus on the chemical weathering processes.

Keywords: soils; pore waters; chemical weathering; secondary minerals; isotope fractionationx

1. Introduction
2. Sample site
3. Methods
4. Results
@4.1. Major element concentrations
@4.2. Lithium and lithium isotope ratios
@4.3. Magnesium and magnesium isotope ratios
@4.4. Silicon and silicon isotope ratios
5. Discussion
@5.1. Elemental ratios
@5.2. Li isotope ratios
@5.3. Mg isotope ratios
@5.4. Si isotope ratios
@5.5. Comparison to river water
@5.6. Isotopic co-behaviour
6. Conclusions
Acknowledgements
References


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